Interesting; it seems you have cracked it.

However, the tolerances on these fittings are very fine. Whilst the rifle is now firing, is it firing accurately and producing groups? Or is the pellet still slightly veering to one side on entry? It is strange that the tap has become worn and I was unaware that by tightening, the tap could be moved. That in turn suggests care must be taken when tightening the tap screw but what is the reference point by which to determine the tightness?!

I don't know how the pellet moves/jumps, in travel, from the tap to the barrel. Thus, is the start of the barrel rather larger and does it then taper inwards, so as to guide the pellet during its flight? Certainly, the tap itself is very slightly tapered inwards at the bottom.

Rgds
A